Saturn rises to the left of the Moon around 8 o'clock. It looks like a bright golden star.

Although it has a small solid core, most of Saturn consists of hydrogen and helium, the two lightest chemical elements. So overall, the planet is less dense than water. That makes Saturn the least-dense planet of all.

Or to be more precise, it's the least-dense planet in our solar system. A few planets in other star systems are even lighter.

An example is a planet discovered last year by the orbiting Kepler observatory. The craft is continuously monitoring about 150,000 stars in search of Earth-like planets in Earth-like orbits. But that's a process that will take several years. In the meantime, Kepler is finding many other planets that are big and orbit close to their parent stars.

In its first few weeks of operation, for example, it found five such planets, which were later confirmed with ground-based telescopes. The planets are many times bigger than Earth, and they're so close to their parent stars that they complete an orbit in just a few days.

One of the planets is known as Kepler 7b. It's about as massive as Saturn. It's so close to its star, though, that it's hotter than molten lava. The heat has caused it to puff up to almost twice Saturn's size. So Kepler 7b is much less dense than Saturn. In fact, the planet is less dense than Styrofoam. Now that's a big gasbag.